1. Field of Invention
The present invention relates to a method for hot-dip zinc galvanizing of steel materials, more particularly to a two-stage hot-dip zinc plating method.
The present invention also relates to rimmed steel, on which a hot-dip zinc plating layer containing Al is provided. The rimmed steel herein is not at all limited by its application and includes, for example, use for general construction, sheet material, plate material and the like. Furthermore, the usual components of the rimmed steel other than Si are 0.3% or less of C and 0.50% or less of Mn.
More specifically, the present invention relates to an improvement of the two-stage hot-dip zinc plating method, such that no failure in appearance occurs on the rimmed steel and the corrosion-resistance of hot-dip plating coating is improved.
2. Description of Related Arts
Heretofore, the general method for improving the corrosion-resistance of hot-dip zinc plated steel has been to increase the coating weight of the plating. In order to increase the coating weight of the plating, pre-treatment prior to the plating may be carried out by subjecting the steel to blasting. Alternatively, the dipping time in a fused zinc bath may be extended. In each case, it is intended to develop a Fe--Zn alloy layer and hence to increase the coating weight of the plating. Nevertheless, improvement of the corrosion-resistance falls short of expectation. Furthermore, the Fe--Zn layer may develop up to the surface of the coating layer, so that a phenomenon referred to as "yellowing" is incurred, which impairs the plating appearance and commercial value of the plated products.
In recent years, not only in the field of continuous hot-dip zinc galvanizing of a strip but also in hot-dip galvanizing of cut sheets, an Al--Zn alloy bath, which is a Zn bath with the addition of Al, has been used to suppress the formation of the Fe--Zn alloy layer and also to improve corrosion resistance. As long as a conventional flux is used, a preferential reaction occurs between the aluminum of the Zn--Al alloy bath and Cl of the flux, with the result that the alloying reaction between the steel and Al--Zn is impeded, thereby generating a phenomenon referred to as the "non-plating".
In order to solve the above described problems, Japanese Unexamined Patent Publication No. Sho 60-125,361, Japanese Examined Patent Publication No. Hei 01-5,110, and Japanese Unexamined Patent Publication No. Hei 03-100,151 propose a special flux which does not impede the formation of an Al--Zn alloy plating.
Japanese Unexamined Patent Publication No. Sho 53-47,055; and Japanese Unexamined Patent Publication No. Hei 05-106,002 propose to add a third element to the Al--Zn alloy bath so as to form an Al--Zn alloy plating by a single dipping.
Japanese Unexamined Patent Publication No. Sho 61-201,767 proposes to form a plating coating by means of hot dipping in a Zn bath without the addition of Al and then to supply Al into the plating coating by means of dipping in an Al--Zn alloy bath. According to this method, the Al--Zn alloy plating layer can be thinly formed by a two-stage plating method. In an example of the above Japanese Unexamined Patent Publication No. Sho 61-201,767, the plating is applied on steel for constructional use (SS41 corresponding to ISO Standard SS400).
Heretofore, the steel material, on which the Al--Zn alloy plating is applied, is not specified in most of the Japanese patent applications, although it may occasionally be specified such as high-tensile steel as in Scope of Claim for Patent (Japanese Unexamined Patent Publication No. Hei 04-311,553) or in Examples (for example in Japanese Unexamined Patent Publication No. Hei 05-106,002, or SPCC in Japanese Unexamined Patent Publication No. 53-47,055).
Previously, hot-dip zinc plating was considered to be applicable to either rimmed steel, semi-killed steel or killed steel ("The Making, Shaping and Treating of Steel", edited by United Steel Corporation, pages 356 and 357 of the Japanese Translation, second edition, third printing).